[exim] ACL question

This may be another RTFM, but I have a difficult time completely understanding the ACL system anyway, so forgive me. I'm trying to define a special rule in the ACL that matches against both sender AND recipient domains. Basically what I want is if they are both a particular domain, reject the message. I.e. if it's from user1@??? AND addressed to user2@???, then block the message. Otherwise, follow the other rules normally. The problem I have is that all the ACL rules I seem to be looking at compare to either the sender's domain or the receiver's domain, but I don't seem to see a way to specifically do what I'm looking for. I'd appreciate some suggestions. I figure I can use ${domain} for the recipient domain, but I don't recall seeing a ${sender_domain} or anything like that.

Thanks!
--- Dan